Timing-based servo (TBS) is a technology developed specifically for linear tape drives in the late '90s. In TBS systems, recorded servo patterns consist of transitions with two different azimuthal slopes, and head position is derived from the relative timing of pulses generated by a narrow head reading the pattern. TBS patterns also allow the encoding of additional longitudinal position (LPOS) information without affecting the generation of the transversal position error signal (PES). This is obtained by shifting transitions from their nominal pattern position, as shown in FIG. 1. In tape systems there are typically available two dedicated servo channels from which LPOS information as well as PES can be derived. The timing-based track-following servo for linear tape systems has been adopted by the linear tape open (LTO) consortium as a standard for the so-called LTO tape drive systems.
Usually, the detection of LPOS information is based on the observation of the shifts of the peaks of the dibit signal samples at the servo-channel output. This approach has the following severe limitations:                a) The A/D converter sampling frequency has to change with the tape velocity if a constant rate expressed in number of samples per micrometer that is independent of tape velocity is desired. If a fixed sampling frequency is adopted, the rate depends on the tape velocity.        b) A consequence of the choice of a fixed sampling rate is that the number of samples per dibit response at the LPOS detector (peak detector) is variable depending on the velocity.        c) No reliable LPOS detection is possible during acceleration and deceleration, i.e., during the phase at which the velocity is changing towards the target velocity.        d) Peak detection is not the optimum detection scheme for the LPOS pattern, which is generated using pulse position modulation (PPM) techniques.        e) There is no possibility to monitor the time evolution of the signal at the output of the servo channel since there is no time reference.        f) There is no possibility to have a measure of reliability of the LPOS detection process.        
In order to support sufficient resolution at high velocities using prior art LPOS asynchronous architecture, illustrated in FIG. 2, higher ADC sampling rates are required. For example, if the highest target velocity is v=12.5 m/s, then a resolution of 0.83 μm is obtained assuming an ADC sampling rate of 15 MHz. Clearly, such a resolution is not adequate when one wants to resolve LPOS pulse position modulation of +/−0.25 μm. In particular, a sampling rate of 250 MHz would be required to obtain a resolution of 0.05 μm using the asynchronous approach.